In order to connect a portable communications device, such as a mobile phone or laptop computer, to a cellular telephony network operating according to for example the GSM and UMTS standards, it is necessary for the device to include a subscriber identity module (SIM) card. SIM cards currently are implemented by universal integrated circuit cards (UICCs). SIM cards are standardised both in size and in the protocols that are used to transfer data to and from the SIM card. A UICC typically includes a CPU (central processing unit), ROM (read only memory), RAM (random access memory), EEPROM (electrically erasable programmable read only memory) and I/O (input/output) circuits. A UICC card will typically include storage of a few hundred kilobytes.
Originally, SIM cards required a supply voltage of 5 volts, although more commonly now the required supply voltage is 3 volts. Some SIM cards require a lower voltage of 1.8 volts.
Battery technology prevalent in mobile communication devices is Carbon-Lithium Cobalt Oxide (C—LiCoO2) battery technology. Batteries constructed using this technology have a discharge cut-off of around 3.1 volts. Since a SIM card needs to be operational in order for a mobile device to be able to communicate via a mobile telephone network, SIM cards are provided with battery power via a voltage upconverter, for instance a buck/boost converter.